The conventional recreational spa tub includes a water-carrying shell, a base, and a series of vertically-oriented support members secured to a base and supporting the shell. The base is often made out of typical construction rails that are very well known and used to construct floor assemblies, such as decks for homes and boat docks. There are, however, several disadvantages with using exposed wood planks for these applications. Wood, if left untreated, can very quickly rot, thus requiring replacement of some if not all of the wood rails. This occurs especially for structures like an outdoor spa tub that are subject to outdoor weather conditions such as rain, snow and sunlight. In addition, wood planks can shrink, creating unsightly and dangerous gaps. Finally, wood is becoming more and more expensive.
Pressure treated lumber is widely used to protect the wood from rotting. However, even pressure treated lumber begins to rot over time with exposure to the elements. In addition, it is recommended by most vendors of pressure treated lumber that a protectant be applied to the wood. This protectant usually must be applied yearly and is therefore time consuming and costly.
Plastic extrusion is a convenient alternative for use in a spa tub base rail as it overcomes the noted problems with wood, however the extrusion process and the plastic extruded spa tub rails themselves still have drawbacks.
Successful extrusion manufacturing requires that every parameter be identified, controlled, and monitored. Some of the variables are based on equipment, others on operating conditions. The variables range from the quality of the die and materials to temperatures and pressures. Some preventative measures can be taken before the manufacturing process even begins. For example, equipment instruments used to monitor temperature, pressure, RPM and amperage should be calibrated twice a year so the readings don't drift over time.
A steady-state process of resin is also essential which begins with how the resin is stored. Typically it needs to be a in a clean, dry area, without being subject to extreme temperature variation. A resin analysis can be conducted to measure the material and record density, melt index, shear rate vs. viscosity data, and tensile strength. Resin that is too dry may not melt and therefore cannot be processed. A change from one batch of resin to another mid-manufacturing process can also result in an altered product. During material lot changes, machine variables have to be monitored more closely in order to make adjustments as needed to maintain efficient extrusion processing and eliminate quality issues with the product.
Typical extrusion problems fall into a few main categories: aesthetic flaws (e.g., pits, black specs, pinholes, drag marks, die lines, sink marks), size variance (which can be intermittent or contiguous); and dimensional variations. The main variables that can occur during the actual process can also be equipment dependent and include: melt pressure, melt temperature, temperature of the barrel, temperature of the die, heater power, cooling power, speed of the screw, the motor load in amps, the speed of the line, die wear or improper design.
When extruding plastic profiles such as a spa tub rail, viability of the end product is usually determined by the cooling of the profile and the ability to hold the part in the correct shape while it is being cooled. It is difficult to cool simple shapes like round pipe and tubing quickly and that difficulty increases when the complexity of the profile increases. Special sophisticated profiles and other complex parts are very difficult to cool uniformly, and if the parts do not cool uniformly warping and bow is the result.
Like most materials, plastics shrink as the temperature of the plastic decreases, but they usually shrink a lot more than other materials. Plastics shrink at one rate when they are in the solid (frozen) state, but they shrink much more when they are still soft or in the molten state. The problem for the profile extruder is controlling this shrinkage when cooling the hot plastic, coming out of the extruder, all the way down to room temperature.
For example, a flat sheet of plastic remains soft as both sides are shrinking at the same rate. Even if one side is cooling faster and shrinking faster the other side is still pliable enough to come along with the other shrinking side. However, once one side cools past the crystalline temperature or its glass transition temperature, two events occur. First, the material stiffens and is no longer pliable enough to follow the other side and the rate of shrinkage goes down significantly. It is as if the stiffened side is no longer shrinking while the other pliable side continues to shrink. Therefore, as the pliable side continues to shrink it is pulling on the stiffened side and causing a bow in the direction of the side that cooled last.
In this example, and in other simple profiles, the part will bow in the direction of the material that cooled last. In more complex profiles the parts may twist, distort, or warp in all types of fashions depending on which sections of the part cooled last.
An additional problem is that plastic is a good thermal insulator and therefore does not transfer heat very fast and does not do it uniformly. Furthermore the thermal conductivity of most plastics is very low at values between 0.1 and 0.30. Attempting to run the plastic profiles faster to increase the length of time the profile has to cool often causes warping, especially with complex hollow shapes having varying wall thicknesses, as well as wood/plastic composites or foamed profiles.
The most effective way to reduce warping is to cool the profile more evenly during the extrusion process by increasing the cooling down time and improve the profile design to make it more symmetric and thinner.
Thus a construction profile for a spa tub base which has improved impact resistance, toughness, UV Resistant, improved surface quality such as gloss is desirable.